DIAGNOSIS This is almost always because the new filter wasn't properly primed or soaked before its first use. Air pockets and carbon dust inside the filter are getting into your water.
FIX You need to flush the filter. Take it out of the pitcher. 1) Hold it under cold tap water with the opening pointed up for 15 seconds. 2) Then, invert it and gently shake it sideways (like a maraca) for a few seconds under the water to dislodge air. 3) Reinstall it in the reservoir, fill the reservoir completely with cold tap water, and let it filter through. Dump that first full pitcher of water. Your second pitcher should taste normal.
IF THAT DIDN'T WORK If the taste persists after two full pitchers, check that the filter is genuine and not a knock-off. Also, ensure you removed the entire plastic wrapper and didn't leave any bits of the seal on it.
HEADS UP Always use cold water in the reservoir. Hot water can damage the filter media and release more carbon fines, making the taste worse.